The conventional gasoline carburetor uses a float to maintain a liquid level in a reservoir slightly below the discharge of the fuel nozzle. Means must be provided to lift the fuel and break the fuel's surface tension before any fuel can be fed into the main air stream on the ambient side of the throttle valve. A main fuel jet is provided to feed an accelerating well or an emulsion chamber. To compensate for a weak fuel supply at low air flows, the jet is sized more largely than theoretically would be required. The air signal is then progressively weakened with air bleeds as the air flow increases.
The float type carburetor is a blending of several different systems. The slow speed idle system uses manifold vacuum to lift fuel from the acceleratinq well and is the predominant system at idle, but as the throttle is opened an idle progression system supports the idle system. Then as the throttle is opened further, the fuel nozzle begins to feed into the main air stream on the ambient side of the throttle plate. As the air demand increases, the idle and progression systems become less predominant and at maximum power the idle system is phased out completely. Since all of the fuel flows through the main fuel jet, the pressure difference across it is less than the air signal, but is directly related to it.
The blending of the systems for the updraft carburetor is closely related to specific engine and manifolding. The feeding of fuel through the fuel nozzle cannot begin until the air velocity is great enough to lift the fuel and keep it in suspension until it crosses the throttle valve; therefore, it becomes a trade-off of air capacity against the proper operation of the low speed, low load operation.